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| 型號： | GPTRM |
| 英文描述： | General-Purpose Timer Reference Manual |
| 中文描述： | 通用定時(shí)器參考手冊 |
| 文件頁數(shù)： | 59/90頁 |
| 文件大小： | 647K |
| 代理商： | GPTRM |
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GPT
REFERENCE MANUAL
APPLICATIONS AND EXAMPLES
MOTOROLA
10-1
SECTION 10APPLICATIONS AND EXAMPLES
Microcontroller applications generally require interfacing to external events and then
provide a response. This response may be based on the timing relationship between
the event and a timing reference. A controller design may also require general-pur-
pose I/O to select certain functions or to monitor the state of sensors such as limit
switches.
Examples in this section show how the GPT can be used in different controller de-
signs.
10.1 Electronic Motor Speed Control
An example of this timing relationship is the tachometer feedback signal in a motor
controller circuit and the controller output signal that drives the motor. An input capture
can automatically record the time of a tachometer pulse as each pulse occurs. This
input capture can be used to determine the velocity and acceleration rate of the motor.
The value of the timer counter is latched into the input capture register each time a
pulse is generated from the tachometer. The velocity can be determined by subtract-
ing two successive input captures. This value multiplied by the time between TCNT
counts gives the time between tach pulses. For example, if the MCU is operating at
16.77 MHz and the prescaler is set to divide by 256, the TCNT will be clocked at a
65536-Hz rate. This gives a time of 15.26 microseconds between each count. A
scheme to drive a motor can be implemented by taking the ratio of the measured ve-
locity in counts and a setpoint velocity in counts. This gives a value that represents the
percentage of error from the setpoint in counts. This percentage of error can be used
by the CPU to compute the value of a control signal used to control the velocity of the
motor.
The tachometer can be any type that produces a digital signal. In
Figure 10-1
an op-
tical tach is used. The number of pulses produced per revolution should be such that
the TCNT does not increment more than 65536 counts at the lowest speed. This keeps
the software from having to consider the effects of the counter rolling over. Also, at the
maximum motor speed, there must be adequate time between tach pulses for the
TCNT to increment. The processor must also have enough time to service the interrupt
before another interrupt occurs.
The output control signal to the motor driver can be a pulse-width modulated (PWM)
signal. In the example, the motor is driven by a switching amplifier. The PWM signal
switches the current on and off through the motor at the PWM signal's periodic rate.
The average motor current is determined by the ratio of on time to off time. The longer
the PWM signal is at logic level one, the higher the average motor current. The per-
centage of error term must be manipulated to take into account motor and load char-
acteristics. The modified error term is used to change the current PWMx value. The
new PWMx value keeps the motor at the proper velocity.
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
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